Kelsey-Lee Barber knew it too.

She was sitting in second spot, just behind surprise early pacesetter and fellow Australian Mackenzie Little after two of six rounds in the world championships final.

As usually happens between throws, she strolled to the fence at Hayward Field to chat to her coach.

"She looked at me and said 'I've got this'," Mike Barber told AAP.

"The crowd went quiet and she was able to get a clap going.

"Just the sound of that stadium was incredible tonight with the clap and everyone focusing on her..

"That's when Kelsey shines.

"Once she did that it was game over."

Barber's monster third-round throw of 66.91m was the best in the world this year and her second- longest of all time, further even than when she won her first world title in Doha three years ago.

Mike Barber was pretty certain no-one else was going to match it and he was right, with American Kara Winger eventually claiming silver with 64.05m.

"Kelsey and I have been in a really strange place leading into this one in that it's the most confident she's ever been coming into a major championship," he said.

"Her body felt really good, she was really settled with her cues and in what she was trying to achieve.

"She just didn't have a (big) distance on the board from a previous competition.

"But other than that, everything we've been doing suggested she was ready to do what she did today.

"One thing I've learnt about Kelsey over the years is that she's almost able to will it to happen.

"She's able to extract every ounce of what she's got on the day."

With one eye on the Birmingham Commonwealth Games, Barber chose to pass on her final two throws, while staying ready to step back in if one of the other competitors was able to do something special in the final round.

As it turned out, they couldn't.

"I definitely didn't get complacent," said Barber, who won her first world title in 2019 with a clutch final-round throw.

"I've been the one to come in at round six and do something special.

"Round six is just a different element.

"You stand at the top of the runway knowing it's your last chance.

"You can honestly do anything in the javelin, you can gain multiple metres from one throw to the next so I was prepared."

Australian Associated Press